Elastin is a protein that is present together with collagen in connective tissue such as the dermis, ligament, tendon, or vascular wall of animals, and in particular mammals. Elastin is normally present in vivo as an insoluble protein having a three-dimensional network structure. It is widely known that hydrolyzing such elastin with an acid or an alkali or treating it with an enzyme gives a water-soluble elastin. Since water-soluble elastin has the ability to retain a large amount of moisture, it is used in cosmetics, in particular as a moisturizing agent (e.g. Patent Publications 1 to 3), and also, together with collagen, as a health food due to cosmetic effects such as giving the skin elasticity (e.g. Patent Publications 4 to 6). Furthermore, it is anticipated that water-soluble elastin will be used in the field of regenerative medicine, such as artificial blood vessels (e.g. Patent Publications 7 to 10).
(Patent Publication 1) JP-A-60-258107 (JP-A denotes a Japanese unexamined patent application publication)
(Patent Publication 2) JP-A-60-181005
(Patent Publication 3) JP-A-2002-205913
(Patent Publication 4) JP-A-6-7092
(Patent Publication 5) JP-A-2005-13123
(Patent Publication 6) JP-A-2005-13124
(Patent Publication 7) JP-B-6-30616 (JP-B denotes a Japanese examined patent application publication)
(Patent Publication 8) JP-A-8-33661
(Patent Publication 9) JP-A-9-173361
(Patent Publication 10) WO 2002/96978
Various methods and means for obtaining water-soluble elastin have been proposed, but there is not yet an adequate method for obtaining a high-purity water-soluble elastin having an appropriate molecular weight. Elastin is extracted from the body tissue of an animal, and in this case it is usual to use animal body tissue that has been subjected to a pretreatment such as removal of unwanted portions or a delipidation operation. The pretreated tissue is dissolved in an acidic liquid containing formic acid or oxalic acid at a predetermined temperature or treated with an enzyme so as to fragment the insoluble elastin contained in the animal body tissue, thus giving a solubilizing liquid in which the water-soluble elastin is dissolved. However, in such a method, there is the problem that, in addition to elastin, collagen and other proteins contained in the animal body tissue are dissolved in the solubilizing liquid in which the water-soluble elastin has dissolved, and the purity of the water-soluble elastin that is finally obtained decreases. Moreover, because the water-soluble elastin that has dissolved in the solubilizing liquid has a high concentration in the solubilizing liquid or has been dissolved in the solubilizing liquid for a long time, the water-soluble elastin molecules might be cut up into low-molecular-weight polypeptides and lose the ability to coacervate in a low temperature zone (e.g. 35° C. to 40° C.). There is also the problem that elastin that has lost the ability to coacervate is not suitable for application in the field of medical materials, etc.
It has been reported that water-soluble α-elastin and β-elastin can be obtained by subjecting purified insoluble elastin to an extraction treatment with hot oxalic acid (Nonpatent Publication 1). However, the molecular weight of the α-elastin reported in Nonpatent Publication 1 is 70,000 and the molecular weight of the β-elastin is 10,000 or less, and these are different from those of the high-purity water-soluble elastin of the present invention, which will be described later. Patent Publication 1 above discloses that an insoluble elastin is decomposed by a protease to give a soluble elastin having a molecular weight of 15,000 to 300,000. However, this elastin has a very broad molecular weight range and contains fragments due to enzymatic decomposition, etc., and it seems unlikely that the purity will be high. Patent Publication 7 above reports that an insoluble elastin is decomposed with pepsin to give a water-soluble elastin having a molecular weight of 8,300 to 640,000, but this is unlikely to have a high purity judging from the amino acid composition (in particular, proline, glycine, alanine, valine). Furthermore, Patent Publication 10 above reports that a water-soluble elastin is obtained by treating an insoluble elastin with hot oxalic acid; it can be anticipated from the amino acid composition that this will have a high purity, and in this publication in order to obtain a biocompatible functional material the water-soluble elastin thus obtained is crosslinked. The amino acid composition of the purified insoluble elastin partially overlaps that of the high-purity water-soluble elastin of the present invention; it is reported to be formed from 80% to 83% proline, glycine, alanine, and valine, 2% to 3% aspartic acid and glutamic acid, 0.7% to 1.0% lysine, histidine, and arginine, and 0.3% to 0.4% desmosine and isodesmosine (e.g. Nonpatent Publication 2).
(Nonpatent Publication 1) Biochimica et Biophysica Acta, 310 (1973) 481-486
(Nonpatent Publication 2) Analytical Biochemistry, 64 (1975) 255-259